1. Field of the Invention
The invention relates to a digital data transmission system providing multipoint communications. Each multipoint communication is effected between a main data transceiver and a plurality of secondary data transceivers. The secondary data transceiver each, in turn, communicate with the main transceiver.
The multipoint communications and the data transceivers used in the present invention are defined in "Recommendation X50" of the International Telegraph and Telephone Consultative Committee (C.C.I.T.T.), Fifth Plenary Assembly, Geneva, December 1972, Green Book, volume VIII, Data Transmission, Published by the International Telecommunications Union, pages 201 through 203.
For a multipoint communication, a plurality of data transceivers which are the so called "data terminal equipment" (D.T.E.) are connected therebetween through the digital links of a digital switching network which have the same digital rate. This plurality of data transceivers comprises a data transceiver called a "main data transceiver", while the other data transceivers are each called a "secondary data transceiver". The main transceiver transmits main data which are delivered to all of the secondary transceivers.
The main transceiver examines in turn, each of the secondary transceivers in accordance with the data transmission method which varies in accordance with the type of the digital network. Consequently, all of the data transmitted from the main transceiver are received in the secondary transceivers. In reverse, the data transmitting means of the one of secondary transceivers is operated only at a predetermined instant in time so that the data transmitting means transmits data to the main transceiver. In other words, each of the secondary transceivers transmits, in turn, only one at a time the data to the main transceiver. These data transfers are effected by means of a multipoint junction unit and the various switching operations between the first order or low rate digital links which convey the multiplexed main and secondary data are accomplished by means of the digital switching unit of a digital frame.
2. Description of the Prior Art
A multipoint digital data transmission system is described, for example, in French Patent No. 2,346,915, filed on Mar. 31, 1976. The French Pat. No. 2,346,915 issued on Sept. 11, 1978. In this system, the component transceiver data are transmitted in the form of recurrent octets on bidirectional digital transmission links having first low rates which are different from but multiples of one another. The low rate links undergo a first time-division multiplexing by means of first multiplexer and demultiplexer units into second order bidirectional digital transmission links having a second rate.
To give an idea of the rates, the first binary rates, for example, are 12.8, 6.4, 3.2 and 0.8 kbit/s and the second binary rate is 64 kbit/s. A 64 kbit/s link may be derived from the time-division multiplexing of a component links at 12.8 kbit/s, b component links at 6.4 kbit/s, c component links at 3.2 kbit/s and d component links at 0.8 kbit/s in accordance with the following relation: EQU 16a+8b+4c+d=80
80 being the number of octets in the recurrent frame of a 64 kbit/s rate link. Thus, the octets belonging at these low rate lines are repeated every fifth octet, every tenth octet, every twentieth and every eightieth octet on the 64 kbit/s frame, respectively.
In accordance with the aforementioned French Patent Application, a multipoint junction unit is connected to the digital switching unit of the frame by a 64 kbit/s bidirectional multiplex transmission link in a similar manner to the connection between each of the first multiplexer and demultiplexer unit and the digital switching unit. The octets of a point-to-point communication are directly transferred from the second rate incoming component link to the correspondent second rate outgoing component link in the frame. The octets of a multipoint communication pass through the digital switching unit once and are transmitted through the associated second rate bidirectional link to the multipoint unit which recopies them in several identical octets which again pass through the digital switching unit and are dispatched to the divers appropriate second rate outgoing component links connected to the multipoint communication transceivers.
The frame according to the French Pat. No. 2,346,915 or, more specifically, the digital switching unit of the frame, may be of a first type disclosed in U.S. Pat. No. 3,952,162, issued Apr. 20, 1976, or of a second type disclosed in U.S. patent application Ser. No. 884,360, filed on Mar. 7, 1978 in the name of Claude A. MOLLERON and Germain G. BROUARD. The essential difference between these two types of digital switching unit results in the fact that the address of the octet or, more specifically, the second part of the octet address--as defined hereinafter--designates the number of the low rate channels or links out of 80 in the second order link frame in accordance with the first type, whereas in accordance with the second type it is determined by the channel number out of the five low rate channels of a second rate frame which may be transmitted solely a low rate channel having the higher low rate of 12.8 kbit/s and by the position number of the octet in this low rate channel.
Taking, for example, the digital switching unit of the first type above, this multiplexes 256 second rate links into eight 2.048 Mbit/s parallel internal multiplex junctions on which the octets are conveyed in parallel.
The octets of the low rate component links each comprises a framing bit. These framing bits form a pseudo-random sequence used to calculate the addresses of the octets in the 64 kbit/s frame in such a way that the digital switching unit effects the first transfer of the multipoint communication octets towards the associated multipoint junction unit. During the octet exchange phase in the multipoint unit which transmits the incoming octets to the digital switching unit, the multipoint unit writes a further pseudo-random sequence into the framing bit location of the incoming octets in order to enable the first multiplexer and demultiplexer units to time-division demultiplex correctly the low rate outgoing digital channels which are transmitted from the digital switching unit.
Thus, according to the French Patent Application No. 2,346,915, each multipoint unit decodes the addresses transmitted by the framing bits in order to effect the appropriate switching operations, i.e. to identify the low first rate octets belonging to each multipoint communication. Furthermore, each first rate octet includes, as is known, a status bit used to identify whether, in this case particularly, a secondary data transceiver is in the transmission phase or not or, more specifically, to identify which secondary data transceiver of a multipoint communication is in communication with the associated main data transceiver. Addressing and identification have the drawback of entailing the complex logical structure of the multipoint junction unit but, more important, generate a relatively lenthy transfer time for each octet.
It will be noted that since the bidirectional digital links are connected to a multipoint junction unit operating at a second rate of 64 kbit/s and the largest common divisor of the octet numbers 5, 10, 20 and 80 of the first rate component links in the 64 kbit/s 80-octet frame is equal to five, a main data octet is only associated with a maximum secondary data number which is equal to four for a multipoint communication accomplished through the multipoint junction unit. This condition limits the possible number of secondary transceivers for each multipoint communication to a low specific value. Consequently, the multipoint digital data transmission system does not offer the requisite operational flexibility needed to adapt the number of accesses to each multipoint junction unit to the actual number of transceivers connected to it.